Solar cells are photovoltaic devices that convert sunlight directly into electrical power. The most common solar cell material is silicon, which is in the form of single or polycrystalline wafers. However, the cost of electricity generated using silicon-based solar cells is higher than the cost of electricity generated by the more traditional methods. Therefore, since early 1970's there has been an effort to reduce cost of solar cells for terrestrial use. One way of reducing the cost of solar cells is to develop low-cost thin film growth techniques that can deposit solar-cell-quality absorber materials on large area substrates and to fabricate these devices using high-throughput, low-cost methods. Group IIB-VIA compounds such as CdTe, Group IBIIIAVIA compounds and amorphous Group IVA materials such as amorphous Si and amorphous Si alloys are important thin film materials that are being developed.
Group IBIIIAVIA compound semiconductors comprising some of the Group IB (Cu, Ag, Au), Group IIIA (B, Al, Ga, In, Tl) and Group VIA (O, S, Se, Te, Po) materials or elements of the periodic table are excellent absorber materials for thin film solar cell structures. Especially, compounds of Cu, In, Ga, Se and S which are generally referred to as CIGS(S), or Cu(In,Ga)(S,Se)2 or CuIn1-xGax (SySe1-y)k, where 0≦x≦1, 0≦y≦1 and k is approximately 2, have already been employed in solar cell structures that yielded conversion efficiencies approaching 20%. Among the family of compounds, best efficiencies have been obtained for those containing both Ga and In, with a Ga amount in the 15-25%. Recently absorbers comprising Al have also been developed and high efficiency solar cells have been demonstrated using such absorbers.
The structure of a conventional Group IBIIIAVIA compound photovoltaic cell such as a Cu(In,Ga,Al)(S,Se,Te)2 thin film solar cell is shown in FIG. 1. The device 10 is fabricated on a substrate 11, such as a sheet of glass, a sheet of metal, an insulating foil or web, or a conductive foil or web. The absorber film 12, which comprises a material in the family of Cu(In,Ga,Al)(S,Se,Te)2, is grown over a conductive layer 13 or a contact layer, which is previously deposited on the substrate 11 and which acts as the electrical ohmic contact to the device. The most commonly used contact layer or conductive layer in the solar cell structure of FIG. 1 is Molybdenum (Mo). If the substrate itself is a properly selected conductive material such as a Mo foil, it is possible not to use a conductive layer 13, since the substrate 11 may then be used as the ohmic contact to the device. The conductive layer 13 may also act as a diffusion barrier in case the metallic foil is reactive. For example, foils comprising materials such as Al, Ni, Cu may be used as substrates provided a barrier such as a Mo layer is deposited on them protecting them from Se or S vapors. The barrier is often deposited on both sides of the foil to protect it well. After the absorber film 12 is grown, a transparent layer 14 such as a CdS, ZnO or CdS/ZnO stack is formed on the absorber film. Radiation 15 enters the device through the transparent layer 14. Metallic grids (not shown) may also be deposited over the transparent layer 14 to reduce the effective series resistance of the device. The preferred electrical type of the absorber film 12 is p-type, and the preferred electrical type of the transparent layer 14 is n-type. However, an n-type absorber and a p-type window layer can also be utilized. The preferred device structure of FIG. 1 is called a “substrate-type” structure. A “superstrate-type” structure can also be constructed by depositing a transparent conductive layer on a transparent superstrate such as glass or transparent polymeric foil, and then depositing the Cu(In,Ga,Al)(S,Se,Te)2 absorber film, and finally forming an ohmic contact to the device by a conductive layer. In this superstrate structure light enters the device from the transparent superstrate side. A variety of materials, deposited by a variety of methods, can be used to provide the various layers of the device shown in FIG. 1.
CdTe solar cell structure is typically a superstrate structure that is obtained by first depositing a transparent conductive layer (TCO) on a transparent substrate such as glass, and then depositing layers of CdS, CdTe and an ohmic contact. The ohmic contact is traditionally a metallic contact such as Ni or an ink deposited material comprising graphite. A small amount of Cu is also traditionally added to the ohmic contact composition to improve its performance. CdTe solar cells with above 16% conversion efficiency have been demonstrated with such structures.
Thin film photovoltaic devices may be manufactured in the form of monolithically integrated modules where electrical interconnection of individual solar cells in a series is achieved on a single substrate, such as a glass sheet, during the film deposition steps and a module with high voltage is obtained. Alternatively thin film solar cells may be manufactured individually and then connected in series, through use of soldering or conductive epoxies just like Si solar cells to obtain high voltage modules. In this case, solar cells often need to be large area, one dimension being more than 1″, typically more than 3″. Such large area requires deposition of finger patterns over the top conducting layer of the solar cell, such as the transparent layer 14 in FIG. 1.
FIGS. 2a and 2b show a top view and a cross-sectional view (taken at location A-A′), respectively, of an exemplary prior art Cu(In,Ga)(Se,S)2 solar cell 20 fabricated on a conductive foil substrate 25 such as a stainless steel foil or an aluminum-based foil. The solar cell 20 has an optional back contact layer 26, an active layer 27, a transparent conductive layer 28, and a finger pattern comprising fingers 21 and a busbar 22. If the conductive foil substrate 25 itself is a good ohmic contact material (such as Mo) there may not be a need for a back contact layer 26. Otherwise a material such as Mo may be used to form a back contact layer 26. As an example, the thicknesses of the transparent conductive layer 28 and the active layer 27 are 500-1000 nm and 1000-2000 nm, respectively. The active layer 27 may comprise an absorber layer such as a Cu(In,Ga)(Se,S)2 and a junction partner such as a CdS buffer layer which lies between the absorber layer and the transparent conductive layer 28. The thickness of the busbar 22 and the fingers 21 may be in the range of 12000-120000 nm, busbar 22 being thicker than the fingers 21. When solar cells with the structure shown in FIGS. 2a and 2b are interconnected, the bottom electrode, or the conductive foil substrate, of one cell is electrically connected to the busbar of the next cell. During this interconnection process ribbons may be soldered onto the busbars and bottom electrodes of the cells or the cells may be laid on each other in a shingled manner so that the bottom electrode of one cell touches the busbar of the next cell. After positioning this way, cells may be pressed together and heat may be applied to assure good contact. It should be appreciated that the thin film structure of FIGS. 2a and 2b is rather fragile because the active layer thickness is only 1000-2000 nm. Physical stress during shingle interconnection or thermal stress generated by heating or soldering processes cause damage to the active layer 27, especially right under the busbar 22, and result in electrical shorts between the busbar 22 and the conductive foil substrate 25 through the damaged active layer. Another shorting path is the exposed edge wall 25a of the conductive foil substrate 25 at the edge region 29. Interconnect soldering materials, conductive epoxies, conductive inks etc. may flow along this exposed edge wall 25a and create an electrical short between the transparent conductive layer 28 and the conductive foil substrate 25. Such shorts reduce yield and deteriorate efficiency of the modules manufactured using thin film solar cells.
In prior work, approaches have been developed to reduce or eliminate shunting effects in thin film structures. U.S. Pat. Nos. 4,590,327 and 4,633,033 discuss some of these approaches, which address possible shunting effects between a busbar of a finger pattern and the underlying active layer comprising an absorber layer. Accordingly, referring to FIG. 2B, these approaches introduce a high resistance layer at the interface 28a between the busbar 22 and the transparent conductive layer 28. Such approaches have certain shortcomings. One problem is the fact that since the high resistance layer is deposited over the transparent conductive layer, its adhesion is controlled by the adhesion of the transparent conductive layer to the active layer as well as the adhesion of the active layer to the back contact layer. When the busbar is formed over the high resistance layer, therefore, its mechanical and electrical stability is a strong function of the mechanical stability of the underlying active layer, which, in thin film structures sometimes cannot support the stress introduced by the thick busbar and annealing and lamination processes. If busbars detach from the substrate due to poor adhesion of the active layer to the back contact, solar cell efficiency suffers.
As the brief review above suggests there is a need to develop device structures and manufacturing approaches to reduce shunting effects in thin film solar cells.